1. Field of the Invention
The present invention pertains to a barrier layer used in the formation of semiconductor devices. The barrier layer has a particular structure, and may be formed in a continuous process to reduce fabrication costs. In the most preferred embodiment, the barrier layer is titanium nitride which is used to line a contact via having a particularly high aspect ratio.
2. Brief Description of the Background Art
In the field of semiconductor device fabrication, particularly with the continuing trend toward smaller device feature sizes, the reliability of electrical contacts has become critical. This reliability is particularly threatened for contacts between aluminum and diffused junctions into single-crystal silicon, where the aluminum and silicon tend to interdiffuse. As is well known in the art, conventional integrated circuit process steps can cause aluminum atoms to diffuse from a metal electrode of pure aluminum into single-crystal silicon to such a depth as to short out a shallow p-n junction in the silicon; this phenomenon is known as junction spiking.
To prevent junction spiking, barrier layers have been introduced between the silicon and the overlying aluminum layer. Typically these barrier layers are formed of refractory metal compound such as titanium tungsten (TiW), or a refractory metal nitride such as titanium nitride (TiN).
U.S. Pat. No. 5,543,357 to Yamada et al., issued Aug. 6, 1996, describes a process for manufacturing a semiconductor device wherein a titanium film is used as an under film for an aluminum alloy film to prevent the device characteristics of the aluminum alloy film from deteriorating. The thickness of the titanium film is set to 10% or less of the thickness of the aluminum alloy film and at most 25 nm. In the case of the aluminum alloy film containing no silicon, the titanium film is set to 5% of less of the thickness of the aluminum alloy film. The aluminum film is formed at a substrate temperature of 200.degree. C. or less by a sputtering process, and when the aluminum film or an aluminum alloy film is to fill a via hole, the substrate is heated to fluidize the aluminum. The pressure during the aluminum film formation and during the fluidization is lower than 10.sup.-7 Torr. A titanium nitride barrier layer may be applied on an interlayered insulating film (or over a titanium layer which has been applied to the insulating film), followed by formation of a titanium film over the titanium nitride film, and finally by formation of the aluminum film over the titanium film. After formation of the titanium nitride barrier layer, the barrier layer is heated to a temperature of about 600.degree. C. to 700.degree. C. in a nitrogen atmosphere using a halogen lamp so that any titanium which is not nitrided will become nitrided. The titanium nitride barrier layer is said to be a poor barrier layer if un-nitrided titanium is present within the layer.
To further improve the performance of TiN barrier layer properties, an oxide has been incorporated at grain boundaries within a titanium nitride film, to increase the ability of the film to prevent the mutual diffusion of silicon and aluminum through the barrier layer. Placement of the oxide at the grain boundaries is known as "oxygen stuffing". U.S. Pat. No. 5,514,908 to Liao et al., issued May 7, 1996, describes an even further improvement in an oxygen-stuffed TiN film, where a titanium silicide layer is formed at the exposed silicon surface, followed by the formation of a titanium oxynitride layer, followed by the titanium nitride layer. In a preferred embodiment of the invention, a porous titanium nitride layer is formed over the titanium metal from which the silicide is to be formed. The wafer is then exposed to an oxygen-bearing atmosphere, to allow oxygen to enter the film. Subsequently, the wafer is rapid thermal annealed to cause silicidation at the silicon-titanium interface and to cause the titanium nitride to densify into a high density film with a titanium oxynitride layer at the silicide/nitride interface.
The Liao patent referenced above describes a technique used to form a relatively low density titanium nitride where the reactive sputtering of titanium nitride is carried out at relatively cool substrate temperatures (on the order of 100.degree. C.) and at relatively weak vacuum conditions (on the order of about 10 mT). This is contrasted with conventional high density titanium nitride which is reactive sputtered at substrate temperatures on the order of 300.degree. C., and at vacuums of "at most" 4 mT to provide the large grain sizes and high density desired in the prior art to provide barrier layer performance.
B. Pecz et al. in an article entitled "Electron microscopy characterization of TiN films on Si, grown by D.C. reactive magnetron sputtering", Thin Solid Films 268 (1995) 57-63, describe the structural characteristics of titanium nitride films deposited by D.C. reactive magnetron sputtering on &lt;001&gt; silicon wafers. In particular, the shape and size of the titanium nitride crystallites were investigated as a function of deposition temperature, substrate bias voltage and nitrogen flow rate (nitrogen content of the process gases). The titanium nitride films are said to exhibit a columnar growth showing preferred orientation along &lt;111&gt; direction. The crystal orientation of the silicon substrate was not seen to affect the mode of crystallite growth. However, when the substrate bias was low, V.sub.b of -40V, intercolumnar voids formed along the grain boundaries. No significant changes in the morphology of the grains were observed over deposition temperatures ranging from room temperature to 550.degree. C. At high flow rates of nitrogen, stoichiometric TiN was the only compound formed, however, at sufficiently low nitrogen flow rates, a film consisting of a mixture of Ti.sub.2 N with a small amount of Ti is formed, and the columnar morphology is said to be absent.
A series of ten sequential layers deposited at different thicknesses, where the growth conditions for the TiN film were slightly varied. For example, the bias voltage was varied from -120V to -125V, and the nitrogen flow rate was varied from about 2.3 sccm to 2.5 sccm (within the nitrogen flow rates where stoichiometric TiN was the only compound formed). Distinct interfaces appeared between the sequential thin layers. However, when the deposition was interrupted (for periods up to 10 minutes) there was no change in the lattice of the TiN grains due to the interruption. The columnar morphology appeared at an early state of growth and was interrupted only at the interfaces where the growth conditions changed.
U.S. Pat. No. 4,944,961 to Lu et al., issued Jul. 31, 1990, describes a process for partially ionized beam deposition of metals or metal alloys on substrates, such as semiconductor wafers. Metal vaporized from a crucible is partially ionized at the crucible exit, and the ionized vapor is drawn to the substrate by an imposed bias. Control of substrate temperature is said to allow non-conformal coverage of stepped surfaces such as trenches or vias. When higher temperatures are used, stepped surfaces are planarized. The examples given are for aluminum deposition, where the non-conformal deposition is carried out with substrate temperatures ranging between about 150.degree. C. and about 200.degree. C., and the planarized deposition is carried out with substrate temperatures ranging between about 250.degree. C. and about 350.degree. C.
S. M. Rossnagel and J. Hopwood describe a technique of combining conventional magnetron sputtering with a high density, inductively coupled RF plasma in the region between the sputtering cathode and the substrate in their 1993 article titled "Metal ion deposition from ionized magnetron sputtering discharge", published in the J. Vac. Sci. Technol. B. Vol. 12, No. 1, January/February 1994. One of the examples given is for titanium nitride film deposition using reactive sputtering, where a titanium cathode is used in combination with a plasma formed from a combination of argon and nitrogen gases.
U.S. patent application Ser. No. 08/511,825 of Xu et al., filed Aug. 7, 1995, assigned to the assignee of the present application, and hereby incorporated by reference in its entirety, describes a method of forming a titanium nitride-comprising barrier layer which acts as a carrier layer. The carrier layer enables the filling of apertures such as vias, holes or trenches of high aspect ratio and the planarization of a conductive film deposited over the carrier layer at reduced temperatures compared to prior art methods.
A "traditionally sputtered" titanium nitride-comprising film or layer is deposited on a substrate by contacting a titanium target with a plasma created from an inert gas such as argon in combination with nitrogen gas. A portion of the titanium sputtered from the target reacts with nitrogen gas which has been activated by the plasma to produce titanium nitride, and the gas phase mixture contacts the substrate to form a layer on the substrate. Although such a traditionally sputtered titanium nitride-comprising layer can act as a wetting layer for hot aluminum fill of contact vias, good fill of the via generally is not achieved at substrate surface temperature of less than about 500.degree. C. To provide for aluminum fill at a lower temperature, Xu et al. (as described in U.S. patent application Ser. No. 08/511,825 now U.S. Pat. No. 5,962,923), developed a technique for creating a titanium nitride-comprising barrier layer which can act as a smooth carrier layer, enabling aluminum to flow over the barrier layer surface at lower temperatures (at temperatures as low as about 350.degree. C., for example). A typical barrier layer described by Xu et al., is a combination of three layers including a first layer of titanium (Ti) deposited over the surface of the via; a second layer of titanium nitride (TiN) is deposited over the surface of the first titanium layer; finally a layer of TiN.sub.x is deposited over the TiN second layer. The three layers are deposited using Ion Metal Plasma (IMP) techniques which are described subsequently herein. Typically the first layer of titanium is approximately 100 .ANG. to 200 .ANG. thick; the second layer of TiN is about 800 .ANG. thick, and the third layer of TiN.sub.x is about 60 .ANG. thick. A good fill of contact vias having 0.25.mu. diameter through holes having an aspect ratio of about 5 was achieved.
U.S. patent application Ser. No. 08/825,216 of Ngan et al., filed Mar. 27, 1997 now U.S. Pat. No. 5,925,225, discloses various process techniques which can be used to control the crystal orientation of a titanium nitride barrier layer as it is deposited. By producing a titanium nitride barrier layer having a high &lt;111&gt; grain orientation content, the &lt;111&gt; grain orientation of an overlying aluminum layer is increased, whereby the electromigration properties of the aluminum are improved.
U.S. patent application Ser. No. 08/824,911 of Ngan et al., filed Mar. 27, 1997 discloses improved Ti/TiN/TiN.sub.x barrier/wetting layer structures which enable the aluminum filling of high aspect vias while providing an aluminum fill exhibiting a high degree of aluminum &lt;111&gt; grain orientation.
Although deposition of a metallic interconnect layer which exhibits a reduced tendency for electromigration and for general diffusion of the metal from the metallic layer is helpful, there is still the problem of the migration of mobile silicon atoms during device fabrication. The use of oxygen stuffing reduces the silicon migration problem, but requires a thermal annealing which is time consuming and increases the cost of the fabrication equipment. It would, then, be highly desirable to have a barrier layer structure which prevents silicon migration without the need for oxygen stuffing and without the need for high temperature annealing of the barrier layer structure (as required for oxygen stuffing and for the formation of a silicide layer).